This invention pertains to clear or pigmented emulsion coatings treated by cation exchange where the emulsion polymers contain hydroxyl or carboxyl groups but are free of amine groups. Said U.S. Ser. No. 824,241 describes a stable catalyst-free acrylic latex composition adapted to cure with amine-aldehyde and/or glycoluril curing agents. The stable reactive coating is based on ion-exchange of a water dispersed mixture of glycoluril resin and acrylic emulsion polymer, where the emulsion polymer contains carboxyl and/or hydroxyl groups but is free of amine groups.
Prior to this invention, glycoluril cured thermosetting emulsions were produced from non-ionic and acidic surfactants and generally required high levels of such surfactants. Anionic salt surfactants cause a detriment to the curing reaction. Further, pigmentation of such glycoluril and emulsion mixtures caused undesirable flocculation during the pigment grind step and subsequent let-down to produce pigmented mixtures.
Prior art U.S. Pat. Nos. 4,442,257 and 4,444,941 teach the use of certain acrylic latexes crosslinked with either tetramenthylol glycoluril (TMGU) or dihydroxy dimethylol ethylene urea (DDEU) for low-temperature curing coatings. Rapid cure is achieved with these products by adding from 1.5% to 3.5% by weight of a 40% solution of paratoluene sulfonic acid where a package pH of about 1.0 to 2.0 results. Stability of these catalyzed products ordinarily is limited to 1-5 days at room temperature, and much less at elevated temperatures. Hence, the acid catalyst must be added at the time of use, which can be dangerous since the acid is quite corrosive. Cured films of such polymer mixture often exhibit certain water sensitivity due to residual catalyst in the cured films. Without the catalyst, however, the coating has no solvent or water resistance unless baked at extremely high temperatures.
Abbey (U.S. Pat. No. 4,525,260) discloses a cationic latex for cathodic electrocoating and specifically includes amine monomers. The reference latex is based on acrylic monomers copolymerized with amine monomers, such as N,N-dimethyl-2-aminoethyl methacrylate, and catalyzed with an azo catalyst. Abbey subsequently treats the cationic latex with an ion exchange resin but to specifically remove amino monomers and amino initiator fragments. Hence, the reference is specifically directed to removing amino fragments introduced in the emulsion process. As noted below, amine latexes are not operative in this invention in that amine groups block coreactivity with a glycoluril cross-linker and inhibit the cure.
It now has been found that stable clear and pigmented emulsion dispersions containing glycoluril-type cross-linkers can be produced by using conventional surfactants, where the mixture is subsequently subjected to cationic exchange to remove the cations from the anionic surfactant as well as from other sources. The emulsion polymer specifically contains hydroxyl and/or carboxyl groups but not amine groups. Amine groups block reactivity, inhibiting cure and render the process of this invention inoperative. In accordance with this invention, the resulting cationic exchange step produces a clear emulsion mixture adapted to cure without the addition of acid catalysts such as p-toluenesulfonic acid. Hence, the disadvantages associated with the use of external acid catalysts are overcome by this invention. Dynamic mechanical analysis further demonstrates that the cure achieved by ion exchange is faster than by acid catalysis. The pigmented or clear mixtures exhibit excellent uninhibited cure with long-term viscosity stability. Accordingly, basic conventional surfactants used as the dispersant enables a dilatent free grind in producing the pigmented emulsion. Upon cation exchange treatment of the glycoluril and emulsion polymer dispersion, the mixture is stable against settling and exhibits no flocculation. In accordance with this invention, the ion-exchange process utilizes a proton-substituted cation exchange resin to remove cations rather than use detrimental external acid catalysts to effect the crosslinking reaction. This process unexpectedly yields a liquid coating with longterm package stability as compared with the acid-catalyzed liquid coating which reacts quickly but typically destabilizes and gels in 1-5 days at room temperature. The improved clear or pigmented emulsion coatings of this invention are indefinitely stable at room temperature yet, upon drying, cure at room temperature to give a fully crosslinked film.